The present invention relates to a mobile radio communication apparatus. More particularly, the present invention relates to a mobile radio communication apparatus provided with an array antenna.
In recent years, a method for mounting an array antenna on a radio communication apparatus is used in order to set directivity of the apparatus in receiving freely. The array antenna is configured with a plurality of antennas. This array antenna adjusts the amplitude and phase and so forth of signals received through the respective antennas, thereby enable to set directivity of the apparatus in receiving freely. In addition, a signal processing section which performs predetermined processing on the received signal multiplies the signals received through the antennas by complex coefficients, thereby adjusts the amplitude and phase and so forth of the signals. Hereinafter, a conventional radio communication apparatus provided with an array antenna is described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating a configuration of a communication system employing a conventional array antenna radio communication apparatus. In FIG. 1, the circumstances are illustrated, in which a base station 11 mounted with a radio communication apparatus provided with a conventional array antenna performs communication with a mobile station 18.
In the base station 11, radio signals from the mobile station 18 are received through a reception antenna 12 and a reception antenna 13. The signals received through the reception antenna 12 and 13 are subjected to down-conversion into either a base frequency band or an intermediate frequency band respectively in a reception radio circuit 14 and 15, and the resultant signal are outputted to a reception signal processing section 16.
In the reception signal processing section 16, the signals outputted from the respective reception radio circuit 14 and 15 are subjected to demodulation processing, thus reception signals are obtained. In this demodulation processing, signals outputted from the reception radio circuit 14 and 15 are also subjected to a processing that the signals possess a specific characteristic (hereinafter referred to as reception directivity) that receives an electromagnetic wave arriving from desired direction strongly. This processing is performed by obtaining an appropriate complex coefficients multiplied by above-described signals. Therefore, it is possible to receive only a required wave while removing interference waves, thus SIR (Signal to Interference Ratio) of the above-described reception signals is kept highly.
However, the specific characteristics of the reception radio circuit 14 and the reception radio circuit 15 are different individually caused by dispersion of characteristics of analog elements such as an amplifier and so forth employed in an internal section thereof. For this reason, signals received through the reception antenna 12 and the reception antenna 13 are influenced caused by each different unknown amplitude fluctuations or a phase rotation or so forth in the respective reception radio circuit 14 and reception radio circuit 15. Consequently, reception signals which are actually obtained due to the multiplication of the complex coefficient in the reception signal processing section 16 include errors caused by the above-described influences, thus the reception signals possess reception directivity different from what is expected first.
In order to prevent the above described phenomenon, it is necessary to adjust the reception radio circuit 14 and the reception radio circuit 15 so that the characteristics of each reception radio circuit is identical with that of each other. However, it is exceedingly difficult to adjust the characteristics of the analog elements such as an amplifier and so forth accurately and invariably on time.
Accordingly, the characteristics of the reception radio circuit 14 and the reception radio circuit 15 are not adjusted, but the respective characteristics of the reception radio circuit 14 and the reception radio circuit 15 are measured beforehand. In the reception signal processing section 16, the adjustment process (hereinafter referred to as calibration) that determines a complex coefficient to be multiplied while taking account of variations of amplitude and a phase of reception signals corresponding to errors of the characteristics of respective reception radio circuits is adopted.
Generally, the calibration is executed before starting communication in order to measure the characteristics of the respective reception radio circuits. Hereinafter, the calibration method is described referring to FIG. 1 again.
A calibration (CAL) radio signal transmission section 19 which generates radio signals for the calibration is placed at a known position for the base station 11. In the base station 11, the radio signal for the calibration transmitted from the calibration radio signal transmission section 19 is received through the reception antenna 12 and the reception antenna 13.
The signals received through the reception antenna 12 and the reception antenna 13 are outputted to the reception signal processing section 16 through respective the reception radio circuit 14 and the reception radio circuit 15 whose settings are switched so as to enable the radio signals for the calibrations to be received.
In the reception signal processing section 16, respective output signals of the reception radio circuit 14 and the reception radio circuit 15 are observed. Then, deviations on the basis of expected values of amplitude and a phase of output signals of respective reception radio circuits 14 and 15 are held in a compensation table as characteristic errors to be compensated on communication. The compensation table is stored in a recording section 17 provided at an internal portion or an outer portion of the reception signal processing section 16.
After completion of the calibration, in the reception signal processing section 16, when a setting is switched so as to demodulate reception signals transmitted from the mobile station 18, ordinary communication is performed. On the ordinary communication, in the reception signal processing section 16, processing is performed so as to compensate characteristic errors of the respective reception radio circuits while referring to contents of the above-described compensation table.
However, there are problems indicated below for the above-described conventional radio communication apparatus provided with the array antenna. Namely, the base station which is mounted with the radio communication apparatus having the above-described conventional array antenna can not perform communication with a mobile station and calibration processing at the same time. For this reason, either one of the communication with the mobile station or the calibration processing is performed preferentially.
However, when the calibration processing is performed preferentially, it is possible to improve the reception directivity in the base station. On the contrary, since the communication with the mobile station should be discontinued in every calibration processing, inconvenience that smooth communication with the mobile station can not be realized occurs.
While when the communication with the mobile station is performed preferentially, it is possible to perform the communication with the mobile station smoothly. On the contrary, since the calibration processing is restricted, inconvenience that reception directivity of the reception signal in the base station deteriorates, occurs.
Thus, in the conventional radio communication apparatus with the array antenna, since the apparatus should make a sacrifice of either one of smooth communication with the mobile station or accuracy of the reception directivity. As a result, there is a problem that it is difficult to realized such apparatus as a practical apparatus.
The present invention is achieved in consideration of such points. The object of the present invention is to provide an array antenna radio communication apparatus that performs calibration processing while continuing communication with a mobile station.
This object is achieved by determining a frequency band used for communication for respective communication ends and a frequency band used for communication of radio signals for the calibration so that each frequency is identical, thereby enabling to demodulate communication signals and calibration signals at the same time.